The present invention relates to phase constellation modulators. In particular, the present invention relates to reduced complexity phase constellation modulators for continuous phase modulation (CPM) communication systems.
A number of modulation schemes are widely used in communication systems. For example, Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), Minimum Shift Keying (MSK), Gaussian Minimum Shift Keying (GMSK) and the like are often used to encode raw data into single-bit or multi-bit symbols for digital transmission. In QPSK, for example, two bits are represented by transmitting a waveform with one of four possible phase offsets (e.g., plus/minus 45 degrees and plus/minus 135 degrees) of a carrier, cos(wt+phase), during each symbol time duration.
Each of the various types of modulation, of course, has its own advantages and its own drawbacks when used in any particular communication system. For example, both BPSK and QPSK modulations exhibit a power spectral density (PSD) with large side lobes that can cause adjacent channel interference (ACI). The effect of ACI can be mitigated by increasing the frequency separation between channels. However, this solution will result in the reduction of the number of frequency channels that may exist within a given frequency band, and thus reduce the information carrying capacity of the frequency band. The utilization of a filtering process to reduce the side lobes, unfortunately, introduces both intersymbol interference (ISI)xe2x80x94interference between the symbols in the same frequency channel, as well as additional system complexity.
Thus, the communication industry has turned to modulation schemes that are by definition bandwidth efficient to begin with. In other words, rather than trying to effect bandwidth efficiency through filtering to remove sidelobes, the communication industry has turned to bandwidth efficient types of modulation. A more bandwidth efficient modulation, such as minimum shift keying (MSK) eliminates step changes in the phase of the transmitted waveform and is therefore denoted as a CPM. Filtered forms of MSK, such as GMSK, will eliminate step changes in the frequency of the transmitted waveform. Thus, for GMSK, a controlled amount of ISI is introduced into the waveform and symbols are transmitted as gradual changes in phase. The result is that the GMSK waveform has a power spectral density (PSD) that falls off extremely quickly, thereby allowing frequency channels to be packed closely together.
GMSK and other CPMs enable the information carrying capacity and bandwidth efficiency (and potential revenues) of a communication system to be increased. Additional details on modulation techniques may be found, for example, in Chapters 6 and 13 of Principles of Analog and Digital Communications, Second Edition, Jerry D. Gibson, Prentice-Hall (1993).
In the past, however, CPMs have been implemented in unduly complex fashion. As an example, past GMSK modulators unnecessarily required hardware to accumulate a running sum of frequency deviation pulses to determine the phase of the modulated waveform. Generally in a digital implementation of a CPM, only a finite number of phase states is possible, but even systems that precompute the phase states have failed to realize several valid simplifying aspects of CPMs that greatly reduce the complexity of the modulator.
Each reduction in complexity not only decreases the cost of the communication system, but increases the reliability, manufacturability, and maintainability of the communication system. A need has long existed in the industry for a reduced complexity phase constellation modulator.
It is one object of the present invention to provide a modulator to generate a CPM.
It is a second object of the present invention to provide a GMSK modulator.
A third object of the present invention is to provide a GMSK modulator with reduced complexity.
A fourth object of the present invention is to provide a GMSK modulator that takes advantage of both the distribution of a nominal phase constellation (NPC), and the NPC itself to reduce the complexity of the modulator.
One or more of the foregoing objects are met in whole or in part by the method for modulation described herein. The method reduces complexity by using fewer than all phase points in an NPC. The method includes the steps of storing an approximate phase constellation (APC) generated from the NPC, identifying the next baseband phase point in the APC using an N-bit input word, and outputting the next baseband phase point.
The APC may include, for example, phase points comprising in-phase and quadrature-phase values representing a subset of the NPC or phase points derived from the NPC. Note, therefore, that the APC is not limited to including only phase points contained in the NPC. In reducing the number of phase points used in the modulation, the present modulator may identify and group phase points differing by less than a phase threshold. The APC may then store a phase point commensurate with the phase points in each group. The phase threshold may be, for example, 3 degrees.
In identifying the next baseband phase point, the method may shift an input bit into an N-bit shift register (with an N-bit output) and apply the N-bit output to a memory that stores the APC. The memory thereby outputs the next baseband phase point. Alternatively, the method may identify the next baseband phase point by applying an input bit to a finite state machine which outputs the next baseband phase point.